Typically, various movement mechanisms are used in a machine tool in order to move a workpiece (an object to be machined) and a tool for machining the workpiece to desired relative positions.
For instance, linear movement mechanisms are provided in X-axis, Y-axis and Z-axis to a support structure of a table on which the workpiece is placed or a support structure of a head to which the tool is attached in order to move the workpiece and/or the tool in three dimensions. Moreover, a rotary movement mechanism is used for changing a posture of the table and/or the head.
Each of the movement mechanisms includes: two relatively movable members (e.g., a guide member and a movement member movable along the guide member); a driving mechanism for moving the two members; and a guide mechanism for securing an accuracy (guiding accuracy) of a moving direction or a movement axis.
Such a guide mechanism is required to have a high guiding accuracy, namely, a geometrical accuracy showing that a linear movement is conducted in a line as straight as possible and a rotational movement is conducted in a circle as perfect as possible. Further, the guide mechanism is required to have a high load capacity, a low friction and a high damping performance.
Specifically, the guiding accuracy of the guide mechanism affects a positioning accuracy of the two relatively movable members and, consequently, affects a profile accuracy of a workpiece to be machined. The low friction of the guide mechanism affects the positioning accuracy along a movement axis and, consequently, affects the profile accuracy of the workpiece to be machined. Moreover, the damping performance affects damping of vibration between the two relatively movable members. Specifically, a damping level of the vibration generating between the tool and the workpiece affects a machined surface roughness of the workpiece.
A hydrostatic pressure guide mechanism may be used in the guide mechanism of the machine tool (see, for instance, Patent Literature 1: JP 2004-58192 A).
In a typical hydrostatic pressure guide mechanism as disclosed in Patent Literature 1, a static pressure chamber (i.e., a concave portion into which an oil for supporting a static pressure load is supplied) is formed on one of a pair of slide surfaces. A static pressure oil is supplied into the static pressure chamber, and a load is transmitted to the other of the slide surfaces by the static pressure. In other words, only the static pressure oil intervenes between the pair of slide surfaces, so that the pair of slide surfaces are in non-contact with each other with a significantly reduced slide resistance.
The above hydrostatic pressure guide mechanism, which lets an oil film constantly intervene between the pair of slide surfaces irrespective of whether the mechanism is still or in motion, can endure a high load and reliably reduce friction.
However, since the hydrostatic pressure guide mechanism is configured to float an object using the oil film, a damping performance of the hydrostatic pressure guide mechanism is limitative. Moreover, the hydrostatic pressure guide mechanism requires a supply device for supplying the static pressure oil for forming the oil film and a recovery device for recovering the static pressure oil. Especially, a typical hydrostatic pressure guide mechanism using a static pressure oil cannot discharge the oil into the outer air unlike an air static pressure bearing using air. Accordingly, the static pressure oil supplied to a static pressure chamber is discharged from an outer periphery to the outside of the guide mechanism. In particular, the hydrostatic pressure guide mechanism, which discharges a huge amount of the static pressure oil as compared with a sliding guide, requires the recovery device for recovering the static pressure oil and returning the static pressure oil to the supply device. Accordingly, the arrangements of devices and pipes associated with the guide mechanism inevitably become complicated.
By the way, a traditional sliding guide mechanism (dynamic pressure guide mechanism) is still often used as a guide mechanism for a machine tool (Patent Literature 2: JP 2008-238397 A).
The sliding guide mechanism has a pair of smooth slide surfaces, one of which is attached with a less-slide member. The slide surfaces are slid while a lubricating oil is supplied therebetween. The pair of slide surfaces are lubricated with the lubricating oil via the less-slide member.
The above sliding guide mechanism, which enables a sliding guide on the basis of a solid-solid contact between the pair of slide surfaces, exhibits improved guiding accuracy and damping performance while being structurally simplified. However, the sliding guide mechanism has a small load capacity and a large friction coefficient, which is particularly increased when the sliding guide mechanism is started and/or driven at a low speed, and thus occasionally fails to smoothly move to affect a positioning accuracy.
In order to smooth the motion of the machine tool, a hydrostatic pressure guide mechanism excellent in terms of low friction may be used as the guide mechanism in place of the typical sliding guide mechanism.
However, even if the typical sliding guide mechanism is simply replaced by the hydrostatic pressure guide mechanism, a desired performance is unlikely to be obtained because of the above-described difference in characteristics.
Accordingly, a hydrostatic-pressure combined sliding guide mechanism, which uses a combination of a typical sliding guide mechanism and a hydrostatic pressure guide mechanism, is suggested (Patent Literature 3: JP 2016-083763 A).
Specifically, a single guide mechanism includes a combination of a sliding guide mechanism and a hydrostatic pressure guide mechanism, allowing for high guiding accuracy and damping performance (i.e., the characteristics of a dynamic pressure guide mechanism) and a high-load endurance (i.e., the characteristics of a hydrostatic pressure guide mechanism).
In such a hydrostatic-pressure combined sliding guide mechanism, the sliding guide mechanism basically receives a load and determines a guiding accuracy and the hydrostatic pressure guide mechanism supplementally receives the load using a supply pressure of a static pressure oil supplied to a static pressure chamber, thereby increasing an allowable load as a whole.
The above arrangement, however, cannot provide the advantages of the combined use, when the hydrostatic pressure guide mechanism fails to be supplied with a sufficient supply pressure of the static pressure oil and thus the load is not supplementally received with the static pressure oil as desired.
In contrast, an excessive supply of the supply pressure to the hydrostatic pressure guide mechanism causes a rise of a portion receiving the hydrostatic pressure, which results in a gap between the pair of slide surfaces of the sliding guide mechanism. The contact between the slide surfaces of the sliding guide mechanism thus becomes insufficient, lowering the guiding accuracy. Further, malfunction and/or damage or the like may be caused.
Accordingly, the supply state of the static pressure oil needs to be suitably adjusted depending on the load received by the guide mechanism. Specifically, for instance, the supply pressure of the static pressure oil should be increased in case of application of a high load and be reduced in case of application of a low load.
For instance, a load acting on the machine tool is basically attributed to a weight of a workpiece to be machined. Accordingly, the weight of the workpiece may be measured in advance to set the supply pressure of the static pressure oil in a control system.
However, in such a case, the workpiece weight needs to be measured in advance. Further, since the weight of the workpiece may be changed with the progress of machining, the adjustment of the supply pressure is unlikely to be always suitable.
Accordingly, a control method of a hydrostatic pressure guide mechanism allowing for estimating a workpiece weight on a driving mechanism side and suitably adjusting the supply of a static pressure oil has been demanded.